Method and apparatus for controlling refrigerant flow in cryogenic systems

ABSTRACT

When the conventional sensor for actuating the pressureresponsive expansion valve in a cryogenic or very low temperature phase-change refrigeration circuit is charged with a limited vapor change of phase-change fluid having a boiling point significantly lower than the boiling point of the refrigerant used in the refrigeration circuit, a startlingly unexpected difference in kind in the sensitivity, efficiency and mode of operation of the circuit as a whole results and some wellrecognized deficiencies of such circuits are eliminated. This same principle is applicable to thermal expansion valves for controlling refrigerant flow in such systems as liquid nitrogen chillers. I presently believe that optimum results are achieved when the sensor system is charged with a limited vapor charge having a boiling point 26* F. to 30* F. lower than that of the refrigerant used in the circuit and when the limited vapor charge is 20 to 40 psi at ambient temperature, the quantity of said limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

United States Patent Webber Feb. 18,1975

[76] Inventor: Robert C. Webber, 8634 Brookville Rd., Indianapolis, Ind.46239 Filed. July 26, 1974 Appl. No: 492,158

Related US. Application Data [63] Continuation-in-part of Serv No.425,181, Dec. [7.

I973, abandoned.

[52] US. Cl 62/115, 62/225, 62/514. 73/3682, 236/92 B, 236/99 [51] Int.Cl. F25b 41/04 [58] Field of Search 62/114, 115, 224, 225, 62/514;73/3682; 236/92 B, 99 R [56] References Cited UNITED STATES PATENTS2,231,163 2/l94l Johnson 236/92 B 3,091,]20 5/l963 Kounousky.,. 73/3682328L075 lO/l966 Smyers, Jr 236/99 R 3,299,7l0 l/l967 Pauli et al 73/36823.367,l3() 2/l968 Owens i 236/99 R 3,797,266 3/1974 Newton 62/224 OTHERPU BLICATIONS Handbook of Automatic Refrigerant Controls" Alco Valve0)., St. Louis, Mo., 1955, pp. 2537.

Refrigeration and Air Conditioning, W. F. Stoecker,

McGrawHill. 1958, pp. 146-147.

Ashrae Guide and Data Book," 1963, American Society of Heating,Refrigerating and Air Conditioning Engineers, Inc., N.Y.. 1963, pp. 28],282, 603, 604.

Primary E.rami/icrWilliam F. ODea Assistant ExaminerPeter D. FergusonAttorney, Agent, or Firm-William R. Coffey [57] ABSTRACT When theconventional sensor for actuating the pressure-responsive expansionvalve in a cryogenic or very low temperature phase-change refrigerationcircuit is charged with a limited vapor change of phase-change fluidhaving a boiling point significantly lower than the boiling point of therefrigerant used in the refrigeration circuit, a startlingly unexpecteddifference in kind in the sensitivity, efficiency and mode of operationof the circuit as a whole results and some wellrecognized deficienciesof such circuits are eliminated. This same principle is applicable tothermal expansion valves for controlling refrigerant flow in suchsystems as liquid nitrogen chillers. I presently believe that optimumresults are achieved when the sensor system is charged with a limitedvapor charge having a boiling point 26 F. to 30 F. lower than that ofthe refrigerant used in the circuit and when the limited vapor charge is20 to 40 psi at ambient temperature, the quantity of said limited vaporcharge being such as to preclude liquefaction thereof at temperatures tobe encountered by said system.

10 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR CONTROLLINGREFRIGERANT FLOW IN CRYOGENIC SYSTEMS This is a continuation-in-partapplication based upon my pending application Ser. No. 425,181 filedDec. 17, 1973, and now abandoned.

The present invention relates to thermo-expansion valve controls forcryogenic refrigeration systems or very low temperature systems havingcapacities as low as 80 F. to 320 F. including mechanical phasechangerefrigeration systems capable of temperatures as low as 250 F. and suchsystems as liquid nitrogen systems capable of temperatures as low as-320 F.

A conventional, single stage, phase-change refrigeration circuitincludes a compressor, a condenser, an evaporator, and conduit means forleading the refrigerant from the compressor, through the condenser, tothe evaporator and thence back to the compressor, an expansion valvebeing connected in the conduit means between the condenser and theevaporator to control the flow of liquid refrigerant to the evaporator.It is customary to use a pressure-responsive expansion valve dominatedby a thermo-responsive, variable-pressure closed system including a bulbsecured in heatexchange relation to that section of the conduit meanswhich leads from the evaporator back to the compressor. It isuniversally customary to charge that closed system with the same kind ofrefrigerant which is used in the refrigeration circuit.

Reference is made to Handbook ofAuromatic Refrigerant Controls publishedby Alco Valve Co., St. Louis, Missouri, 1955, pages 25-37; Refrigerationand Air- Condilioning by W. F. Stoeker, McGraw-Hill, 1958, pages 146,147; and ASHRAE Guide and Data Book 1963 published by the AmericanSociety of Heating, Refrigeration and Air-Conditioning Engineers, Inc.,New York, 1963, pages 281, 282, 603, 604. Reference is made also to US.Pat. Nos. 2,231,163 issued Feb. 1941 to Johnson in Class 236, subclass92 B and 3,091,120 issued May 1963 to Kounousky in Class 236, sub-class(fluid digest).

These above-mentioned prior art references discuss the use ofthermo-expansion valves which respond to the temperature of therefrigerant gas leaving the evaporator and to the pressure in theevaporator. Three forces govern the valves operation. Those three forcesare the pressure created in the valve by the remote bulb containing therefrigerant and which is attached to the refrigerant line leaving theevaporator, the evaporator pressure, and the equivalent pressure of thesuperheat spring which is contained in the valve. An increase in heatload on the evaporator increases the superheat of the refrigerant gasleaving the evaporator and this, in turn, heats the refrigerant withinthe sensing bulb of the expansion valve causing the expansion valve tomove in an opening direction. A decrease in the heat load on theevaporator decreases the superheat of the refrigerant gas leaving theevaporator and causes the thermostatic expansion valve to move in aclosing direction.

While much is known about such expansion valves and the refrigerantcharges in the valves for relatively high temperature refrigerationsystems, as evidenced by the above-identified references, very little isknown about the use of such valves for extremely low temperatures,sometimes referred to as cryogenic temperatures, in the range of 80F. to320 F. The aboveidentified references do discuss the use of refrigerantsin the control expansion valve which are different from the refrigerantsin the system being controlled, and they even discuss liquid crosscharges. Such prior art references do not, however, even contemplate theextremely low temperatures which are the subject of the presentinvention, nor do they contemplate limited vapor charges which are thesubject of the present invention. 1n fact, the Alco Valve Companyhandbook identified above, at pages 34 and 35, specifies the need for adrop of liquid in the remote bulb for proper control. This is incontrast to the present invention in which the refrigerant charge on theexpansion valve bulb is a limited vapor charge applied at a pressure of20 to 40 psi at ambient temperature, the quantity ofthe limited vaporcharge being such as to preclude liquefaction thereof at temperatures tobe encountered by the refrigeration system. In other words, in thesystems of the present invention, the refrigerant vapor in the controlvalve could never be in liquid form.

I have noted that conventionally-charged expansion valves tend to besluggish and that sometimes they will fail to close as early as theyshould so that liquid refrigerant floods back to the compressor withdeleterious effects or simply fail altogether in very low temperaturerefrigeration systems.

The primary object of the present invention, then, is to render thecontrol for expansion valves for cryogenic systems more sensitive andmore certain to close in response to slight elevations of temperature inthe effluent from the evaporator in such systems.

To the accomplishment of the above and related objects, my invention maybe embodied in the form illustrated in the accompanying drawing,attention being called to the fact, however, that the drawing isillustrative only, and that change may be made in the specificconstruction illustrated and described, and/or in the nature andsequence of the steps set forth herein, so long as the scope of theappended claims is not violated.

The single FIGURE forming a part hereof is a diagrammatic illustrationof a single stage, phase-change refrigeration system in which myinvention may be used.

Referring more particularly to the drawing, it will be seen that l haveillustrated a conventional compressor 10, a water-cooled condenser 11,an evaporator 12, and conduit means including a section 13 leading fromthe compressor 10 to the condenser 11, a section 14, 15 leading from thecondenser 11 to an evaporator 12 and a section 16 leading from theevaporator back to the compressor 10. A pressure-responsive expansionvalve 17 is formed to establish a chamber having a movable wall 18, anda capillary tube 19 connects the interior of that chamber with a bulb 20which is secured in any suitable manner in heat-exchanging relation tothe exterior of the conduit section 16 close to its connection to theevaporator 12.

A charge of phase-change fluid of suitable characteristics fills thesystem 10, ll, 12, 13, 14, 15, 17, 16. The compressor 10 delivers hot,gaseous fluid to the condenser 11 where the fluid is cooled and changedto the liquid phase and flows thence through the conduit sections 14 and15 to the valve 17. When the valve is open, as it will be whenever thepressure in the chamber and impressed upon the movable wall 18 is abovea predetermined value, liquid refrigerant will flow to the evaporator 12where it will undergo a phase change.

The gas emanating from the evaporator 12 when the system is demandingcooling, will be quite warm and will hold up the temperature of thefluid in the bulb 20 so that pressure within the valve chamber willremain relatively high and the valve 17 will remain open. When thedemands upon the evaporator are reduced so that the rate of heatabsorption from the evaporator drops, the refrigerant emerging from theevaporator will be reduced in temperature. That temperature drop will beapplied to the bulb 20 whereby the pressure of the fluid in the bulb 20,tubing 19 and chamber of the valve 17 will drop, ultimately to close thevalve 17 until the demands upon the evaporator 12 again increase. Toften, however, the bulb 20 will become so cold that all of the fluid inthe sensing system will be liquefied with a resultant sharp drop of thepressure in the closed system whereby all control over the valve 17 islost.

I have found that, if the fluid charged into the system 20, 19 is aphase-change fluid having a boiling point significantly below that ofthe refrigerant flowing in the refrigeration system, not only is thepossibility of a complete phase change in the control system eliminated,but also the control system for the valve 17 is rendered more sensitivethroughout its range of operation.

As is set forth in DuPonts Technical Bulletin RT48 LOW-TEMPERATUREREFRIGERANTS, widely used refrigerants include the following, listedwith their chemical constitutions and boiling points:

the refrigeration circuit in combination with different refrigerants inthe sensingsystem. For instance, when I add 1 1% of R22 refrigerant toR-l2 refrigerant and charge the thermo-sensing system with that mixture.then when a conventional expansion valve is fitted into the circuitunder the domination of the closed system so charged, the expansionvalve will perform and oper ate satisfactorily and very efficiently atF. below the boiling point of the said refrigerant R-l2.

Similarly, if I add 8% of R502 refrigerant to R22 refrigerant in theclosed system and apply the system so charged to an R22 expansion valve,the circuit will operate satisfactorily in a circuit charged with R22refrigerant, at F. below the boiling point of the R22 refrigerant.

Of course, still lower temperatures in the refrigeration system may beobtained by adjusting the thermostatic expansion valve spring, thusincreasing the tension of that spring.

While the invention has been illustrated in a singlestage refrigerationcircuit, it will be understood that it is equally applicable to acascade system of two or more stages. For optimum results, the charge inthe closed sensing system should consist of an azeotropic mixture.

1 have found that, using such an azeotropic charge, a conventionalthermostatic expansion valve will oper- Boiling Point, F.

Refrigerant Chemical Constitution Refrigerant l2 CCl. ,F Refrigerant llSC ClF Refrigerant 22 CHClF CHF, Refrigerant 23/116 Refrigerant 170.Ethane 2 6 Refrigerant 503 Refrigerant 23/13 Refrigerant 1150, EthyleneC H Refrigerant l4 CF Refrigerant 50. Methane CH Tentative nomenclatureassignment. All two component refrigerant combinations listed in thetable are azeotropes,

In addition, oxygen has a boiling point of 297.4 F., nitrogen has aboiling point temperature of 320.6 F., hydrogen has a boiling pointtemperature of 422.9 F. and helium has a boiling point temperature of450.4 F.

As stated above, it is universal practice to use an Rl 2 refrigerant inthe sensing system and in the refrigeration circuit of an Rl2refrigeration system. Similarly, R22 refrigerant will be used in bothparts of an R22 system, R502 refrigerant will be used in the maincircuit and in the closed control system of an R502 circuit, etc.

I have discovered that changing the refrigerant charge within thethermo-sensing element or bulb of a system of the character illustratedwill radically improve the automatic operation of the refrigerationsystem and that flood-back of the refrigerant causing the compressor tomalfunction, frost-back and wide fluctuate in refrigeration systemshaving temperature ranges from 350 F to 250 F. without damage to thethermostatic expansion valve.

Using an azeotropic charge of 87% Rl refrigerant and 13% Rl refrigerant,the thermostatic expansion valve will function satisfactorily inrefrigeration systems having temperature ranges from 350 F. to 250 F.

It is noted that the solid components of the disclosed system do notdiffer from those of a conventional phase-change refrigeration system'But in such a system, the refrigerant charge or charges must beconstrued to be elements of the system, since the function of such asystem (to absorb or to emit heat) is absolutely dependent upon thepresence of such refrigerants and the presence of such refrigerants isabsolutely essential to the accomplishment of a useful function by therecited elements in combination. Thus, the abovedescribed change of therefrigerant charge in the c'ontrol system is equivalent to a change in amechanical element of a recited combination.

Although they have no bearing on the invention herein disclosed, I havediagrammatically indicated the conventional pressure-actuated valve 22for controlling cooling water flow to the condenser 11, receiver 23,valve 24 with fusible plug 25, drier 26 and vibration dampers 27customarily present in simple, phasechange refrigeration systems.

While the above-given examples are not necessarily for very lowtemperature refrigeration systems, they are examples of systems I havemodified to obtain very satisfactory results by placing a refrigerant inthe thermo-sensing system which has a boiling point significantly lowerthan the refrigerant in the refrigeration circuit. For very lowtemperatures or cryogenic temperatures, however, I have discovered thatthe charge in the thermo-sensing system must be first, a limited vaporcharge, second, a refrigerant having a boiling point to 30 F. below theboiling point of the refrigerant in the circuit of the system beingcontrolled, and third, the limited vapor charge must be between 20 to 40psi at ambient temperature. The quantity of the lim ited vapor chargemust be such as to preclude liquefaction thereof at temperatures to beencountered by the system.

For instance, Ethane has a boiling point of l26.9 F. while Ethylene hasa boiling point of l55.0 F. I may build a satisfactory system inaccordance with the present invention using Ethane in the refrigerationcircuit and using a limited vapor charge of Ethylene in the control loopor in the bulb and housing of the expansion valve at a pressure ofapproximately 30 psi at ambient temperature. The 30 psi pressure atambient temperature would still provide 17 lbs. pressure at l40 F. suchthat a 15 psi valve would still be open. Such valves have adjustmentscrews and consequently, the Ethylene may be charged into the controlloop at a pressure of 30 psi i 10 psi at ambient. Importantly, theEthylene is charged into the bulb and housing of the thermo-expansionvalve in a limited vapor charge.

In order to provide the refrigerant in the control loop with apreferable boiling point 26 to 30 F. lower than or cooler than theboiling point of the refrigerant in the circuit of the basic system,i.e., flowing through the evaporator, I may provide a cross charge ofvapor. That is, I may combine, for instance, the vapors of tworefrigerants to reduce the boiling point of the vapor charge in the bulband housing of the thermoexpansion valve. Since Ethylene has a boilingpoint of l55.0 F. or 28 F. lower than the boiling point of Ethane, I donot have to mix any other lower boiling point refrigerant with theEthylene unless, for some reason, I want the boiling point difference tobe something greater than 28 F.

I can take standard thermostatic expansion valves, manufactured bycompanies such as Alco, Sporlan, Singer, Automatic Controls, Detroit(American Standard), Parker-Hannifin and others, and charge the sensingbulbs or elements and bellows of such valves with a limited vapor chargeof a special mixture of refrigerants to cause the valve to modulate therefrigeration capacity of the system and not overload its compressor(when starting at ambient) and maintain this condition until thetemperature within the system has been lowered to, for instance, atemperature as low as F. This is accomplished by the special refrigerantvapor mixture in the sensing element and bellows of the valve and theamount of pressure with which the sensing element and baffle arecharged. When the refrigeration system so modified is first energized orput into operation, the suction pressure will not exceed 25 psi, thuspreventing overloading the compressor.

One example of my present invention, therefore, may be a cryogenicrefrigeration circuit charged with Ethane having a boiling point ofl27.5 F. with the baffle and sensor of the thermo-expansion valve chargedwith a limited vapor charge of Ethylene having a boiling point ofl55.0F., the vapor charge being applied at a pressure of 20 to 40 psi atambient temperature and with the quantity of the limited vapor chargebeing such as to preclude liquetication thereof at temperatures to beencountered by the system. The boiling point difference of such asystem,.which would be quite satisfactory, would be 225 F.

I presently believe that the preferred boiling point difference shouldbe somewhere in the range of 26 F. to 30 F., i.e., that the boilingpoint of the refrigerant vapor in the sensor and baffle of thethermo-expansion valve should be 26 to 30 F. lower than the boilingpoint of the refrigerant flowing through the evaporator.

While, to this point, I have discussed refrigeration systems includingcompressors and evaporators, it will be appreciated thatthermo-expansion valves are also applied to refrigeration systems of thetype in which a refrigerant, such as liquid nitrogen, is released toflow through evaporator coils. In such systems, the expansion valve isdisposed between the source of the refrigerant and the evaporator tocontrol the flow of the refrigerant through the evaporator. For such aliquid nitrogen system, I may charge the sensor and bellows of thecontrol valve with a limited vapor charge of nitrogen, which has aboiling point of 320.6 F. and hydrogen, which has a boiling pointof422.9 F., so that the vapor charge will have a boiling point 26 to 30F. lower than the boiling point of nitrogen. Just as I do for mechanicalrefrigeration systems, I limit the quantity of the limited vapor chargeto preclude liquefaction, and I provide the charge at a pressure of 20to 40 psi at ambient temperature.

I claim:

1. A method of controlling refrigerant flow in a cryogenic refrigerationcircuit including a compressor, a condensor, an evaporator, conduitmeans connected to lead refrigerant from the compressor to thecondensor, thence to the evaporator, and thence back to the compressor,and a charge of phase-change refrigerant in said circuit, which includesthe steps of providing a pressure-responsive valve to controlrefrigerant flow through said conduit means, providing athermoresponsive variable-pressure system operatively con nected toactuate said valve, said system including a bulb arranged inheat-exchanging relation with a remote point in said conduit means, andcharging said system with a limited vapor charge of a refrigerant havinga boiling point significantly lower than the boiling point of therefrigerant in the said refrigeration circuit, the limited vapor chargebeing 20 to 40 psi at ambient temperature, and the quantity of saidliquid vapor charge being such as to preclude liquefaction thereof attemperatures to be encountered by said system.

2. The method of claim 1 in which the boiling point of the refrigerantvapor used to charge the variablepressure system is approximately 20 to30 F. lower than the boiling point of the refrigerant used in saidcircult.

3. The method of claim 2 in which the refrigerant used to charge thevariable-pressure system has a boiling point 26 F. to 30 F. lower thanthe boiling point of the refrigerant used in said circuit.

4. The method of claim 3 in which the limited vapor charge is applied atapproximately 30 psi at ambient temperature.

5. A cryogenic refrigeration circuit including a compressor, acondenser, an expansion valve having a movable valve head, anevaporator. conduit means for conducting refrigerant from saidcompressor to said condenser, thence to said expansion valve, thence tosaid evaporator and thence back to said compressor, thus establishing acircuit, a charge of phasechange refrigerant in said circuit, and meansfor controlling refrigerant flow through said valve comprising a closedcontrol system including a chamber in said valve having a movable walloperatively associated with said valve head, a bulb held inheat-exchanging association with a section of said conduit means throughwhich refrigerant flows from said evaporator back to said compressor, acapillary tube providing open communication between said chamber andsaid bulb, and in which the improvement comprises, in said closedsystem, a limited vapor charge of phase-change fluid having a boilingpoint significantly lower than the boiling point of the phase-changerefrigerant in said refrigeration circuit, the quantity of said limitedvapor charge being such as to preclude liquefaction thereof attemperatures to be encountered by said system, said limited vapor chargehaving a pressure of 20 to 40 psi at ambient temperature.

6. The invention of claim Sin which the limited vapor charge has aboiling point approximately 20 to 30 F. lower than the boiling point ofthe refrigerant in said circuit.

7. The invention of claim 6 in which the limited vapor charge has aboiling point approximately 26 to 30 F. lower than the boiling point ofthe refrigerant in said circuit.

8. A method for controlling flow of refrigerant through an evaporator ofa cryogenic cooler including the steps of providing apressure-responsive valve to control refrigerant flow through saidevaporator, providing a thermo-responsive variable-pressure systemoperatively connected to actuate said valve, said system including abulb arranged in heat-exchanging relation with the exhaust side of saidevaporator, and charging said system with a limited vapor charge of arefrigerant having a boiling point significantly lower than the boilingpoint of the refrigerant flowing through said evaporator, the limitedvapor charge being 20 to 40 psi at ambient temperature, and the quantityof said liquid vapor charge being such as to preclude liquefactionthereof at temperatures to be encountered by said system.

9. The method of claim 8 in which the boiling point of the refrigerantvapor used to charge the variablepressure system is approximately 20 to30 F. lower than the boiling point of the refrigerant flowing throughthe evaporator.

10. The method of claim 9 in which the limited vapor charge is appliedat approximately 30 psi at ambient temperature.

{ENZYME STATES PATENT OFFICE (IERTiFHJATE OF CORRECTION FAR 40 3,866,430

DATE; February 18 1975 Ei iV'Etfl 'JRifl Robert C. Webber 3? iscertified m em. appea s n the above-identified patent and that saidLetters Patent a? hen .1;, tazz'ecefl as shown b low In the Abstract,line 4, "vapor change" should be vapor charge Column 6, line 10, "-l27.5F." should be -l26.9 F. 1 line 16, "liquefaction" is misspelled; line19, "22.5 F."

should be 28.l F. line 27, after "compressors" insert condensers line48, (claim 1, line 3) "condenser" is misspelled; line 49 (claim 1, line4) "condenser" is misspelled.

Signed and sealed this 27th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C I- ASON Commissioner of Patents AttestingOfficer and Trademarks

1. A method of controlling refrigerant flow in a cryogenic refrigerationcircuit including a compressor, a condensor, an evaporator, conduitmeans connected to lead refrigerant from the compressor to thecondensor, thence to the evaporator, and thence back to the compressor,and a charge of phase-change refrigerant in said circuit, which includesthe steps of providing a pressure-responsive valve to controlrefrigerant flow through said conduit means, providing athermo-responsive variablepressure system operatively connected toactuate said valve, said system including a bulb arranged inheat-exchanging relation with a remote point in said conduit means, andcharging said system with a limited vapor charge of a refrigerant havinga boiling point significantly lower than the boiling point of therefrigerant in the said refrigeration circuit, the limited vapor chargebeing 20 to 40 psi at ambient temperature, and the quantity of saidliquid vapor charge being such as to preclude liquefaction thereof attemperatures to be encountered by said system.
 2. The method of claim 1in which the boiling point of the refrigerant vapor used to charge thevariable-pressure system is approximately 20* to 30* F. lower than theboiling point of the refrigerant used in said circuit.
 3. The method ofclaim 2 in which the refrigerant used to charge the variable-pressuresystem has a boiling point 26* F. to 30* F. lower than the boiling pointof the refrigerant used in said circuit.
 4. The method of claim 3 inwhich the limited vapor charge is applied at approximately 30 psi atambient temperature.
 5. A cryogenic refrigeration circuit including acompressor, a condenser, an expansion valve having a movable valve head,an evaporator, conduit means for conducting refrigerant from saidcompressor to said condenser, thence to said expansion valve, thence Tosaid evaporator and thence back to said compressor, thus establishing acircuit, a charge of phasechange refrigerant in said circuit, and meansfor controlling refrigerant flow through said valve comprising a closedcontrol system including a chamber in said valve having a movable walloperatively associated with said valve head, a bulb held inheat-exchanging association with a section of said conduit means throughwhich refrigerant flows from said evaporator back to said compressor, acapillary tube providing open communication between said chamber andsaid bulb, and in which the improvement comprises, in said closedsystem, a limited vapor charge of phase-change fluid having a boilingpoint significantly lower than the boiling point of the phase-changerefrigerant in said refrigeration circuit, the quantity of said limitedvapor charge being such as to preclude liquefaction thereof attemperatures to be encountered by said system, said limited vapor chargehaving a pressure of 20 to 40 psi at ambient temperature.
 6. Theinvention of claim 5 in which the limited vapor charge has a boilingpoint approximately 20* to 30* F. lower than the boiling point of therefrigerant in said circuit.
 7. The invention of claim 6 in which thelimited vapor charge has a boiling point approximately 26* to 30* F.lower than the boiling point of the refrigerant in said circuit.
 8. Amethod for controlling flow of refrigerant through an evaporator of acryogenic cooler including the steps of providing a pressure-responsivevalve to control refrigerant flow through said evaporator, providing athermo-responsive variable-pressure system operatively connected toactuate said valve, said system including a bulb arranged inheat-exchanging relation with the exhaust side of said evaporator, andcharging said system with a limited vapor charge of a refrigerant havinga boiling point significantly lower than the boiling point of therefrigerant flowing through said evaporator, the limited vapor chargebeing 20 to 40 psi at ambient temperature, and the quantity of saidliquid vapor charge being such as to preclude liquefaction thereof attemperatures to be encountered by said system.
 9. The method of claim 8in which the boiling point of the refrigerant vapor used to charge thevariable-pressure system is approximately 20* to 30* F. lower than theboiling point of the refrigerant flowing through the evaporator.
 10. Themethod of claim 9 in which the limited vapor charge is applied atapproximately 30 psi at ambient temperature.